Typical waveguide tube filters used in microwave and millimeter-wave bands are realized by using a resonator structure including a metallic waveguide tube formed in a drawn structure. This type of the filters has a drawback in larger dimensions although it is superior in the performance thereof.
Thus, as recited in JP Patent Application 10-82184, a pseudo waveguide tube band-pass filter is devised which has a side wall of the waveguide tube configured by metallic via-holes in a dielectric substrate. As a practical example, FIGS. 9A and 9B show the schematic structure of a filter 10 having a four-stage configuration. FIG. 9A is a perspective view thereof, whereas FIG. 9B is a top plan view thereof. Referring to FIG. 9A, a top conductor 2 is formed on one of the surfaces of the dielectric substrate 1, whereas a bottom conductor 3 is formed on the opposing surface thereof. Via-holes 4 connecting together the top conductor 2 and the bottom conductor 3 are formed in two rows along the signal transfer direction. The spacing “a” between adjacent via-holes is equal to or below ½ of the in-tube wavelength. This structure is construed as a pseudo waveguide tube having a waveguide tube cross section defined by the thickness of the dielectric and the spacing “b” between the two arranged rows of the via-holes 4. Pairs of via-holes 5 are also formed in the waveguide tube to configure resonators having cavity lengths of L1, L2, L3 and L4. By suitably selecting the spacing “c” between the via-holes 5 forming a pair, frequencies other than the resonant frequency can be effectively reflected. On the other hand, a signal in the resonant frequency passes therethrough to achieve a desired filter function. In this filter, the dimensions of the filter are reduced down to about 1/(√ε) compared to a waveguide tube having a hollow interior (c is the relative permittivity of the dielectric).
On the other hand, a filter is often used which is configured by using a micro-strip line on a dielectric substrate. This filter has relatively smaller dimensions and can be connected to a planar circuit, such as an integrated circuit, by wire bonding, thereby allowing the filter to be mounted in a high-frequency module with ease.
Sometimes it is desirable for the above waveguide tube filter to have smaller dimensions. For example, the dimensions of the microwave or millimeter-wave integrated circuit formed on a semiconductor device are of around 5 mm square at a maximum. Accordingly, if a small-size multi-chip module is to be implemented by using an integrated circuit, it is generally important to reduce the dimensions of passive components such as filters. In addition, it is generally difficult to connect the filter to a planar circuit. Thus, a filter is desired which can be mounted and connected with ease and without enlarging the dimensions and adding a particular conversion circuit.
On the other hand, the filter using the micro-strip line sometimes assumes a characteristic change upon mounting the filter in a package structure. This results from the fact that the electromagnetic field in the micro-strip line is distributed up to the top portion thereof and can thus be susceptible to the effects of attaching a cap member thereto.
In the connection structure using the wire bonding technique, especially in the higher frequency range such as millimeter wave band, there arises a characteristic change caused by variation of the bonding wire length or by variation of the parasitic inductance component determined by the bonding wire length. Such a characteristic change is not negligible, and becomes a factor of reducing the product yield in a mass production. For solving this problem, a flip-chip mounting technique has been developed wherein the millimeter-wave semiconductor integrated circuit is mounted with face-down mounting onto a mounting board and connected thereto by using bumps. This technique is described, for example, in “IEEE International Solid-State Circuits Symposium, Digest” pp. 324–325, 2000, by K. Maruhashi et al. When the flip-chip mounting technique is applied, the connection between each element and the mounting board is implemented by a relatively short distance (200 micrometers or less), whereby the influence by the parasitic inductance component in the wire bonding technique becomes negligible. For applying the flip-chip mounting technique to the filter as well, the filter should have a terminal adapted to a coplanar waveguide, which is generally used for connection between elements, and should have a structure wherein the face-down mounting minimally changes the filter characteristic, and thus such a filter has been strongly desired.